In recent years, a wireless local area network (wireless LAN) system based on the IEEE802.11 standards has been widely spread. A wireless communication device provided in the wireless LAN system performs a communication through a radio channel in accordance with a CSMA/CA (carrier sense multiple access with collision avoidance) method. In order to prevent a collision from being generated through a transmission of plurality of signals to the same radio channel at the same time, the wireless communication device occupies the radio channel before performing the communication. The wireless communication device sets an NAV (network allocation vector) for a certain time period in order to occupy the radio channel. For the time period in which the NAV is set, the wireless communication device setting the NAV performs the communication through the radio channel and the other wireless communication device restrains the communication.
The wireless communication device is mounted on a portable terminal such as a mobile personal computer powered by a battery. In order to provide the function of the wireless LAN without giving an inconvenience to a user, therefore, it is important to reduce power consumption of the wireless communication device as greatly as possible.
In the IEEE802.11a/b/g standards, SISO (single input single output) is employed for usage of an antenna for performing communication. However, the IEEE802.11n standard, which is being developed, is compatible with MIMO (multi input multi output) for usage of an antenna for performing communication. For this reason, a wireless communication device compatible with MIMO can implement a higher speed and higher stable wireless communication, although power consumption is increased. Accordingly, it is more important in the wireless LAN system to reduce the power consumption of the wireless communication device.
As a technique for reducing the power consumption of the wireless communication device, the following technique has been reported. Specifically, when a signal indicating that a destination address is other wireless communication devices is received, a time required for a communication of the signal is calculated from a frame size described in a PHY (physical layer) header of the received signal and a frame transmitting speed, and a receiving operation of a receiver unit of the wireless communication device is suspended assuming that a radio channel is occupied by the other wireless communication devices during the calculated time. An example of such system is disclosed in JP-A-2005-303585 (counterpart U.S. publication is: U.S. Pat. No. 7,382,764 B2).
The IEEE802.11n standards are planned to be compatible with a Long NAV. The Long NAV is defined by IEEE802.11 Task Group n (TGn) as a NAV for occupying a radio channel for a longer period than the NAV based on the existing IEEE802.11 standards.
The wireless communication device setting the Long NAV can transmit a signal (CF_End signal: Contention Free End) for suspending the occupancy of the radio channel, thereby suspending the occupancy of the radio channel differently from the case in which the conventional NAV is set.
For example, in the case in which the wireless communication device suspends the receiving operation of the receiver unit of the wireless LAN for a period in which the Long NAV is set to reduce the power consumption, the wireless communication device cannot receive the CF_End signal and cannot detect that the occupancy of the radio channel is suspended even if the CF_End signal is transmitted from another wireless communication device to suspend the occupancy of the radio channel in the middle of the suspension of the receiving operation of the receiver unit in the wireless LAN.
Irrespective of the non-occupancy of the radio channel, therefore, the wireless communication device suspends the receiving operation of the receiver unit of the wireless LAN to restrain the transmission of the signal. Moreover, there is a possibility that the radio channel might be further occupied by the other wireless communication device while the wireless communication device restrains the transmission of the signal. In this case, the wireless communication device has a problem in that a signal cannot be transmitted and a transmitting delay is thus increased for a set period of the Long NAV or a longer period through the other wireless communication device.